Synonyms for Jejune (same or very similar meaning)
WordNet sense 1 (lacking in nutritive value):
insubstantial
WordNet sense 2 (detrimental to physical or moral well-being):
unwholesome
WordNet sense 3 (displaying or suggesting a lack of maturity):
adolescent, juvenile, puerile
WordNet sense 4 (characteristic of a lack of maturity):
immature
WordNet sense 5 (lacking interest or significance):
insipid
From the ODE community, based on WordNet
add/edit